1. Field of the Invention
The present invention relates to a packet transmitting technique and particularly to a packet network including a ring type topology, a packet repeater forming this network, and a packet repeating method.
2. Description of the Related Art
At present, in the wide area ETHERNET® services which are served by the communication companies (so-called type 1 carrier) having their own fundamental facilities required for providing the services, a network is constituted in the form in which many layer 2 switch devices (hereinafter referred to as L2SW devices) are connected as the exchange nodes and various topologies are also used in accordance with application modes.
In order to determine the topology of the network to be constituted, reliability and cost of the network as a whole are very important elements. Particularly, a ring type network is somewhat inferior to a mesh type network but is often employed because reliability is rather high and economical merit can be attained with fewer requirements, such as transmission links, among the nodes forming the network and transmission link interfaces of nodes.
In the ring type network, the reliability thereof is guaranteed with its redundancy function with which the communications can be continued by changing the transfer route of the frames (packets) even if a fault occurs in a part of the transmission links forming the ring.
However, if only the L2SW devices are connected like a ring, a loop is formed in the route for transmission of the data frames (MAC frames). In the case where the MAC frame which has been input to the L2SW device under this condition is the broadcast frame, this frame is increased up to a plurality of frames each time when the frame passes the node. Thereby, traffic congestion that may be termed a broadcast storm occurs each time when this frame circulates the loop. Finally, as a result, the normal data traffic on the ring may be interfered with and the network is likely to cease operation.
Accordingly, a certain means will be required in order to avoid formation of a loop path when the L2SW devices are connected like a ring. Moreover, a highly reliable network cannot be attained if it is impossible that the route can be altered to maintain data traffic by detouring a fault generating location and quickly restart communications, when a fault such as breakdown of a link is generated among the nodes in the ring network.
As explained above, various problems are still left unsolved in the ring type network. For example, sufficient consideration must be taken for loop path, switching of route when a fault occurs, and influence on data traffic which is not related directly to the fault.
Next, the existing ring redundant system in the ETHERNET® will be explained.
The redundant system in relation to reliability of network has been discussed mainly aimed at a system based on the ring type network in each transmission system of SONET (Synchronous Optical NETwork) or SDH (Synchronous Digital Hierarchy) and a system based on a star type network in ETHERNET®.
However, the following system is known as the ring redundant system in ordinary ETHERNET® service. This system has been individually developed by a vendor as the function just suitable for wide area ETHERNET® for communication companies that are requesting to form a ring type network with ETHERNET®, which can be realized at a low cost.
FIG. 13 is a diagram for explaining the redundant system of the existing ring type network.
In FIG. 13, node A71 to node F76 as the L2SW devices form a ring type network using the transmission links 21 to 26 respectively. Node A71 and node B72 are connected with transmission link 21 between port P1 of node A71 and port P2 of node B72. Here, the port and the transmission link are constituted to enable two-way communications.
Moreover, port P3 of each node is connected respectively with terminals 31 to 36, which are assumed to have MAC addresses of X, Y, Z, U, V, and W. Only one port P3 is shown, as the port for connection of the terminal of each node, to each node, for simplicity of the drawing. However, in general, a plurality of terminal's connection ports are also provided for connecting a plurality of terminals.
In this structure, one of nodes A71 to F76 is designated as a master node. In FIG. 13, node A71 is designated as the master node. This master node is manually set by a network administrator. One of the two ports P1, P2 belonging to the ring transmission link is called the primary port and the other, the secondary port. In the example of FIG. 13, port P1 of node A71 is the primary port and port P2 is the secondary port.
The nodes B72 to F76 on the ring other than the master node are called the transit nodes and the ports P1, P2 belonging to the ring are usually in the forwarding state.
The master node transmits health check packets to the ring at an interval of several seconds from its own primary port and the transit node sequentially transfers the received health check packets to the neighboring nodes. The master node recognizes that the ring is in the normal state when the health check packets are received by the secondary port of the master node after making a turn through the ring.
Moreover, the secondary port of the master node is usually capable of receiving only the health check packets in order to prevent the MAC frame from being maintained in the closed loop state having no exit and being in the locking state not receiving any other data traffic. Operation under the normal state of the ring has been explained above.
FIG. 14 is a diagram (No. 2) for explaining the redundant system of the existing ring type network.
Operation when a fault is generated will be explained below, considering an example where a link fault is generated due to breakdown or the like of the transmission link between node C73 and node D74 as illustrated in FIG. 14.
If a link fault is generated, the health check packet transmitted from the master node A71 cannot make a turn through the ring transmission link and cannot return to the master node A71 itself. Moreover, node C73, node D74 having detected the link fault, transmits the fault notifying packet (trap packet) to the master node from the port in the opposite side of the port having detected the fault. In the example of FIG. 14, node C73 transmits the fault notifying packet from port P2, while node D74, from port P1.
The master node A71 changes, upon reception of the fault notifying packet at the primary port P1 or secondary port P2 thereof, the secondary port which has been in the blocking state to the forwarding state. Thereby, the communication link up to node D74 from node C73 is acquired through the master node A71, enabling communications among all nodes.
However, in this timing, since the frames transferred on the ring are switched in accordance with the FDB (Forwarding Data Base) which has been held by each node before generation of a fault, the frames passing node C73 to node D74 before generation of a fault are transferred directly toward the fault generating section and are finally lost as a result. Namely, communication ceases until the FDB of the node is cleared.
In order to avoid such an event, master node A71 transmits the FDB flash packet instructing clearing of the FDB held by the node to all transit nodes immediately after the secondary port P2 is set to the forwarding state. Accordingly, a new communication link can be learned to enable communications by once clearing the FDB holding the transit nodes on the ring.
Operation, when a fault is restored by a maintenance person, of the network will be explained below considering an example in which a link fault between node C73 and node D74 in the fault generating section has been recovered to the normal state.
Owing to recovery of a fault, the health check packet from master node A71 can be transmitted through node C73 to node D74. Therefore, master node A71 can receive the health check packet with its own secondary port P2 and then recognize that the ring has returned to the normal state. Subsequently, master node A71 changes secondary port P2 to the blocking state from the forwarding state. Accordingly, the communication link between node A71 and node F76 enters the communication blocking state, requesting the clearing of the FDB in each transit node with the same reason explained above.
Moreover, the master node A71 transmits the packets to all transmit nodes in order to notify that port P2 is in the blocking state to the transit state and to instruct clear of the FDB itself.
With the operations explained above, reliability of the ring type network can be enhanced in the redundant system. However, in this redundant system, since the master node exists within the ring just as it is fixed and all operations are performed with a certain control packet from the master node, a problem also arises. For example, the communication cease time becomes longer as the number of nodes forming the ring type network increases.
A technique for quickly switching the rings with which the nodes forming the duplicated ring type networks transmit and receive packets using the remaining time of the health check packets in order to avoid a fault in the transmission link or the like has also been disclosed (for example, refer to patent document JP-A No. 2003-234747
The redundant system of the related art explained above has the following problems.
Namely, in the redundant system of the related art, the node which has detected a fault such as link-off or the like transmits the fault notifying packet to the master node during the period until communication has recovered from occurrence of a fault and the master node having received such a packet transitions its own secondary port to the forwarding state from the blocking state and transmits the FDB flash packet to the transit nodes forming the ring type network.
Each transit node having received this FDB flash packet learns again the transfer link of the frames to be transferred by clearing the FDB being held. Accordingly, the transfer link of the frame is changed and communication is started again.
As explained above, the procedures of many stages are required until communication can be started again and therefore the communication cease time becomes longer when the procedures increase. Moreover, the FDB of each node must be cleared to change the transfer link of the frames, but if the FDBs of all nodes on the ring are cleared at one time, a large amount flooding is generated momentarily.
Such flooding is generated by the following operations of each node having cleared the FDB.
For example, in FIG. 13, when terminal 31 transmits data to terminal 36, the frame transmitted by terminal 31 includes X as the transmitting source address (SA) and W as the destination address (DA).
Node A71 having received this frame from port P3 registers first the SA:X and port P3 to the FDB under the corresponding state. Moreover, in order to determine the port for transmitting this frame, the FDB is searched using the DA as the search key and when such FDB is hit, this frame is transferred to the corresponding port.
If any FDB is not hit, this frame is transferred to all ports (port P1, P2) other than the port having received this frame (in this case, port P3).
Because of the flooding of node A71, each node receives this frame.
Each node registers, as explained above, the SA:X and the received port to the FDB under the corresponding state. Moreover, in order to determine the port for transmitting this frame, the FDB is searched using the DA as the search key. When a certain FDB is hit, this frame is transferred to the corresponding port.
If any FDB is not hit, this frame is transferred to all ports other than the port having received this frame. Here, it is assumed that the FDB of node F76 connected with the terminal 36 of the DA:W is not hit. This means that there is no entry having the address W in the FDB because the FDB is cleared or the frame that has the SA:W is not received for a certain period.
In this case, node B72 receives this frame from port P2, for example, and transfers it to ports P1, P3. Moreover, node F76 receives this frame from port P2 and transfers it to ports P1, P3. The frame transmitted from port P3 of node F76 is received by terminal 36 because the DA is the address thereof.
With a series of these operations, entry of the address X is registered to the FDB of each node and the port corresponding to this address respectively becomes node A71: P3, node B72: P2, node C73: P2, node D74: P2, node E75: P2, node F76: P2. This operation is considered as that the FDB learns the MAC address.
When terminal 36 connects, for example, node F76 transmits, under this state, the frame of DA:X to terminal 31, node F76 having received this frame searches the FDB using the X as the search key and obtains the corresponding port P2 and transfers this frame to this port P2. In the same manner, the frame is transferred among the nodes and when this frame is received with port P1 of node A71, the frame is output to port 3 corresponding to address X with the search of the FDB and then transferred to terminal 31. Simultaneously when this frame is transferred, the FDB of each node learns, with a series of transfer processes, correspondence between the address W of terminal W36 as the transmitting source and the port having received the frame and generates entry of address W.
As explained above, when entry is not included in the FDB, flooding is generated and traffic increases. Particularly, after the FDB is cleared, namely after all entries of the FDB are erased, it is apparent that the traffic greatly increases in accordance with the number of terminals connected to the node and the number of nodes cleared, resulting in the problem that a sudden increase in the traffic allows an increase in the link capacity giving influence on the data traffic of the users not related in direct to a fault.
Moreover, since the master node playing the principal role in the ring redundant system of the related art is set as it is fixed, clearing of the FDB is also required even after the fault is repaired, resulting in the problem that congestion of the network is generated because of packet loss and flooding, which should not be generated intrinsically.